Enable automatic light-sleep on esp8266

[kzyapkov]

Currently, mongoose_poll() is invoked every 100 ms, regardless of whether there are events to be handled. This practically makes using the light-sleep functionality of esp8266 impossible with mongoose-os.

Please consider making this possible. It seems it is enough to stop the hardware watchdog and schedule s_mg_poll_tmr for the maximum interval of 0x689D0 when there's nothing going on.

Also here: https://forum.mongoose-os.com/discussion/1423/esp8266-automatic-light-sleep-mode